One million watts of alternating current. When the array is operating at STC, its MWAC is the MWP x the performance ratio.

Performance ratio

%

Ratio of actual output of a solar generating system to the rated peak output of its solar array under prevailing solar radiation conditions [ref chapter 14 of the book]

Output figures and energy ratings

Megawatt hours

MWh

Energy produced by a power plant delivering one MW for one hour. Almost always rated in terms of AC for utility-scale systems

Megawatt hours per annum

MWh/y

Number of megawatt hours delivered by a generating station over the course of a year (a good comparative measure for solar systems, because seasonal output can vary substantially, but year-to-year variations are relatively small)

Load factor or Capacity utilisation

%

Ratio of the electrical energy produced by a generating unit relative to the energy that could have been produced at continuous full-power operation during the same period (typically 12% to 28% for PV plant, depending on location)

Solar duty

MWh/y/

MWP

Number of MWh per annum for each MWP of rated capacity (gives the output and revenue potential of a project). Effectively = load factor ÷ performance ratio.

Availability

% or h/y

Percentage of the time that electricity generating plant is available to produce electricity (free from downtime). Not to be confused with capacity factor

Other glossary items

For solar module testing, defined in international standards; insolation intensity of 1kWm-2 normal to the plane of the solar module at ambient temperature of 25°C and light spectrum equivalent to AM1.5